Sagging breasts and breastfeeding don't go together.
Let's take some major facts here and start analyzing them to put things in their proper order.
   What causes breasts' changes?   The breasts' anatomy and physiology gives us the most solid basis of explanation about  the structure of breasts and the possible ways this can be affected.
  There is a muscular layer over which the gland stands being constituted by tubular elements, connective tissue and fat,  covered by the skin.
As we women know, breasts can change a bit during our period, and they get back to normal again.
  We also know that weight gain or loss has an impact on our breasts.
  The first kind of change is hormonally  induced, while the second one is directly related to the adipose fat.
  During pregnancy, breasts change and this is something NO WOMAN can avoid.
  Pregnancy hormones are responsible for the breasts' enlargement, and their skin's stretching which is a necessary preparative mechanism  aiming at enabling the woman's body to produce milk when the baby comes.
  The breasts will be prepared for breastfeeding no-matter if you decide to breastfeed or not.
  The changes will occur anyway.
The exact pattern of these changes can't be quite predicted though.
  As Dr .
Sears says on his book about Breastfeeding,  "...
some women feel that their breasts are fuller after pregnancy and breastfeeding and some feel that they are smaller or lower...
"  Social "ideals" in our culture  - being mainly expressed by fashion models and celebrities-  stipulate a young woman's well contoured breasts as the perfect ones.
  But most women's breast don't ever look like the "ideal" which doesn't mean they're not sexy and beautiful.
Besides, "Barbie was never pregnant" as accurately Dr Sears quotes.
Maternal breasts take on a rounder and fuller shape but that doesn't mean they're sagging or going to sag.